I put together some instructions of how to embed multiple Google calendars on Google Sites sites. I included screenshots and thought it looked pretty nice, plus I got some positive feedback for it. Unfortunately, Google seems to have changed something about how Sites interprets its calendar widget and made the hack I figure out not work anymore.
My attention was drawn to this by a comment today from a reader who had followed my instructions and received an error. Both his site and mine were complaining about chaining multiple site URLs like Google does in its own embedded calendar customizer. I noted the problem and said I would look into it later. Tonight was later.
This Google question indicated that somebody else was having a similar problem, but did not provide a solution. This chain of posts from the Google Calendar help group presented a solution similar to my original post and discussed a GUI solution from Google that I could not find in Chrome or IE 8.
Then I found this Google Sites help question that provides a work around. I am not entirely happy with it, but it is better than the error I was getting and gets the job done. You can find the instructions here. The trick is to insert a widget that lets you insert the calendar inside of it.